A delay in issuing full preliminary results from the vote in the vast central African state could further complicate an election already marred by deadly violence, logistical problems and allegations of fraud.

"We want to keep to the date of December 6, but we've had some logistical problems, these have hampered us," electoral body spokesperson Mathieu Mpita told Reuters by telephone.

"If we don't have the maximum initial tallies, we will release partial results," he said.

The electoral commission set the deadline for a full preliminary count of December 6, the fifth anniversary of President Joseph Kabila's inauguration, and the day the opposition says marks the end of his constitutional term.

UN-led diplomatic efforts are under way to allow a delay to the results if needed, according to sources. Partial preliminary results issued so far, representing nearly 70% of the ballots cast, give Kabila a 10-point lead over his chief rival, Etienne Tshisekedi.

At least 18 people have been killed in election-related violence so far, according to US-based Human Rights Watch, and a senior member of Kabila's camp said the government will have to call in the army if protests become "too chaotic".

There was a heavy security presence on the streets of the capital on Monday night, and some residents on Monday piled into boats to cross the Congo River into neighbouring Congo Republic, fearing violence.

The first locally organised and funded election since the official end of years of war in 2003 was meant to offer hope of greater stability in the mineral-rich, crisis-riddled giant.

But fears are mounting that a rejection of the results will pave the way for further bloodshed.